In the past, columnar or extended pinning centers could be created in a superconducting thin film only through irradiation with a high energy beam of neutrons or heavy ions (see L. Civale et al., Physical Review Letters 67 No. 5, 648, 1991). According to this method, a material damaged by irradiation with a high energy beam of neutrons or heavy ions is arrayed regularly to form extremely effective columnar pinning centers, namely columnar or extended defects which enhance its critical current density and critical magnetic field. Theoretically, it has been proven by A. Buzdin and M. Daumens (see Physica C, 294, 297, 1998) that a highest pinning effect is achieved if the columnar defect is cross-sectionally in the form of an elongate ellipse, namely if it is in the form of a thin stripe. In order to form such a columnar or extended defect, it is necessary to irradiate a substrate with a heavy ion beam at an angle lower than 90 degrees to its surface.
It has now been known that pinning centers can also be formed in a superconducting thin film by photolithography and lift-off techniques to create a regular array of dots or holes (see M. Baert et al., Physical Review Letters, 74, 3269, 1995).
The abovementioned method of driving a high energy beam of neutrons or heavy ions into is recognized to be extremely useful in making a superconducting thin film with high critical current density, but it requires an accelerator for high energy particles and thus has the problem that its practice is very costly.
Columnar defects having a striped cross section and formed by irradiating a substrate with heavy ions at a low angle have their axes inclined to the c-axis of a superconducting thin film and also deviated from a direction in which a magnetic field is applied generally. Then, for defects to be pinned, it is necessary to tilt the direction of magnetic flux, which causes the pinning free energy to be decreased, however. This is the reason why defects of this type are only effective in a small magnetic field.
On the other hand, a regular array of dots or holes formed by utilizing photolithography or lift-off is not much effective for the reasons as follows. First, their size is larger than 0.1 μm and their length is longer than the superconducting coherence length. Second, their surface density is small and the distance between adjacent dots or holes is 1 μm or more. Under these conditions, it is only in a magnetic field as low as 1 mT that the critical current density can be increased.